High intensity discharge lamps normally include an arc discharge tube having a fill of sodium, mercury, and an inert gas with an electrode sealed into opposite ends thereof. This arc discharge tube is sealed within an outer envelope which ordinarily has an attached base formed to connect to an energizing source. Moreover, it is known that high intensity discharge lamps require a higher starting voltage than an ordinary low pressure lamp.
One known technique for starting high intensity discharge lamps includes the utilization of a special high voltage producing ballast apparatus. Another known technique is to embed a starting electrode in one end of the arc tube and to position the starting electrode adjacent one of the electrodes of the arc tube. However, special high voltage ballast apparatus undesirably adds to the cost of the light source. Also, starting electrodes sealed into one end of the arc tube tend to develop an accumulation of condensed metal halide salts which leads to undesired electrolysis at or near the main electrode and failure of the arc tube seal.
Another arrangement for starting high intensity discharge lamps is set forth in U.S. Pat. No. 4,322,658 issued to Minarczyk on Mar. 30, 1982. Therein, an arc tube having an electrode sealed into each end is positioned within an outer sealed envelope. The electrodes of the arc tube are embedded in a support as is a starting probe which is positioned immediately adjacent the arc tube. A base is affixed to the outer sealed envelope and formed for connection to an AC potential source while an electronic pulsing circuit is located within the base. The pulsing circuit provides a pulse potential during the positive and negative half-cycle of the applied AC potential and these positive and negative pulse potentials are applied to the starting probe until a discharge is established within the lamp. Thereafter, application of the positive and negative pulse potentials to the starting probe is discontinued.
Although the above-described apparatus has been employed with varying degrees of success, it has been found that certain disadvantages do exist and such apparatus does leave something to be desired. More specifically, it is known that the appearance of a negative potential, such as the negative half-cycle of an AC potential, on a starting electrode or probe associated with a discharge lamp which includes sodium tends to cause not only sodium loss from the lamp but undesired electrolysis as well.
As set forth on Pages 266-269 of a book entitled "Electric Discharge Lamps" by John F. Waymouth, M. I. T. Press 1971, an arc lamp having a fill which includes sodium tends to exhibit sodium loss whenever a negative potential appears in the vicinity of the external surface of the arc discharge tube. Briefly, the probe adjacent the arc tube receives a strong flux of ultraviolet light from the arc tube which causes photoelectron emission. Also, the surface of the arc tube tends toward a positive charge which attracts the photoelectrons caused by the negative potential on the probe. Moreover, the photoelectrons accumulating on the outer surface of the arc tube undesirably attract the sodium ions which pass through the arc tube and combine with the photoelectrons. Thus, sodium loss as well as electrolysis deleteriously affect the discharge lamp.
One approach suggesting a technique for reducing sodium loss in a discharge lamp is set forth in U.S. Pat. No. 4,281,274 issued to Bechard et al on July 28, 1981. Therein, a glass sleeve surrounds the arc tube and is connected to a potential which is positive relative to the arc tube. Thus, the glass sleeve acts as a shield trapping ultraviolet light. However, such structures obviously tend to increase cost while reducing manufacturing efficiency.
Known approaches attempting to reduce the above-described undesirable results are also illustrated in the prior art FIGS. 1, 2, and 3. In FIG. 1, a positive coefficient resistor "R", capacitor "C" and thermal protector "T" are series connected across an AC potential source. A glow bottle "G" and the primary of a transformer shunt the capacitor "C" while the transformer secondary is coupled to a starting probe. Similarly FIG. 2 has a positive coeffiecient resistor R and a combined overvoltage and thermal protector series connected and coupled across an AC potential source. A capacitor C and a transformer primary winding shunt the thermal protector with the transformer secondary coupled to a starting probe. However, in both examples, FIGS. 1 and 2, a negative-going half-cycle of the AC potential source is applied to the starting probe causing the above-described undesired sodium loss and electrolysis development.
Additionally, it has been found that rearrangement of circuitry components does little or no good in so far as alterations in probe potential is concerned. For example, FIG. 3 illustrates a configuration wherein a resistor "R", capacitor "C" and thermal switch "T" are series connected across an AC potential source; a spark gap and transformer primary winding are shunted across the capacitor "C" and the transformer secondary is coupled to a starting probe. However, it is clear that the negative half-cycle of an AC potential source will still undesirably appear at the starting probe causing undesired sodium loss and electrolysis.